CN110161033B

CN110161033B - Optical contrast enhancement for defect inspection

Info

Publication number: CN110161033B
Application number: CN201910113011.3A
Authority: CN
Inventors: Z·本埃泽尔
Original assignee: Camtek Ltd
Current assignee: Camtek Ltd
Priority date: 2018-02-13
Filing date: 2019-02-13
Publication date: 2024-11-19
Anticipated expiration: 2039-02-13
Also published as: TW201940866A; TW202424470A; JP2019179026A; US20190251686A1; TWI847976B; US11055836B2; KR20190098084A; CN110161033A

Abstract

The present application discloses optical contrast enhancement for defect inspection. An inspection system and method for inspecting an object. The method may include acquiring a defocused image of a region of the object; and processing the defocused image of the region to find a phase shift between light paths associated with certain neighboring points of the region. The phase shift may indicate a defect. Acquisition of the defocused image may include illuminating the region with a radiation beam, which may be spatially coherent and collimated when impinging on the region. Illumination may include passing the radiation beam through an aperture defined by an aperture stop, which may be located in an aperture stop plane. The size of the aperture may be a fraction of the size of the aperture stop.

Description

Optical contrast enhancement for defect inspection

Cross reference

The present application claims priority from U.S. provisional patent 62/629,761, having a filing date of 2018, 2, 13.

Background

The semiconductor is inspected for defects by two-dimensional (2D) optical imaging with enhanced contrast of the defects.

Some defects are not characterized by differences in reflectivity or transmissivity compared to the surrounding environment and are therefore difficult to detect using standard bright-field or dark-field imaging.

Such defects may have the property of small topographical variations or local stresses affecting the local refractive index.

Methods using phase contrast imaging typically enhance the contrast of defects in 2D images.

This approach, such as DIC (differential interference contrast), requires the insertion of special optical elements into the optical illumination and imaging path.

Brief description of the invention

The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the accompanying drawings; wherein fig. 1 is an example of a system; FIG. 2 is an example of a system; FIG. 3 includes an example of an aperture stop; FIG. 4 shows an example of a portion of a system; FIG. 5 shows an example of the differences between interference patterns; and figure 6 shows an example of a method.

SUMMARY

A method for inspecting an object may be provided, which may include: acquiring a defocused image of a region of an object; and processing the defocused image of the region to find phase shifts between optical paths associated with certain neighboring points of the region; wherein the phase shift may be indicative of a defect. The acquisition of the defocused image may include illuminating the region with a radiation beam, which may be spatially coherent and collimated when impinging on the region. Illumination may include passing the radiation beam through an aperture defined by an aperture stop, which may lie in an aperture stop plane. The size of the aperture may be a fraction of the size of the aperture stop.

The acquisition of a defocused image of the region may include sensing, by a sensor, sensed radiation reflected by the region; wherein the sensed radiation may comprise an interference pattern formed between radiation reflected from different points of the region; wherein the different points of the area may include some neighboring points and additional points.

The method may include sensing a difference between an interference pattern associated with certain neighboring points and an interference pattern associated with additional points.

The small portion may not exceed ten percent.

The aperture may be located at the center of the aperture stop.

The aperture may be located outside the center of the aperture stop.

The aperture stop may be brought into the illumination path prior to acquisition of the defocused image, and may be removed from the illumination path after acquisition of the defocused image.

The method may include acquiring a plurality of defocused images of the region under different defocusing conditions.

The method may include selecting a selected defocus condition of the different defocus conditions and using the selected defocus condition to acquire a defocused image of a region of the other object.

The selection may be based on a contrast associated with the phase shift.

An inspection system may be provided, which may include: an imager that may be constructed and arranged to acquire a defocused image of a region of an object; and a processor that may be constructed and arranged to process the defocused image of the region to find a phase shift between optical paths associated with certain neighboring points of the region. The phase shift may be indicative of a defect. The imager may include illumination optics that may be constructed and arranged to illuminate the region with a radiation beam that may be spatially coherent and collimated when impinging on the region. The illumination optics may include an aperture stop, which may include an aperture. The illumination optics may be constructed and arranged to pass the radiation beam through an aperture before reaching the region. The size of the aperture may be a fraction of the size of the aperture stop.

The imager may include a sensor that may be constructed and arranged to sense sensed radiation reflected by the area; wherein the sensed radiation may comprise an interference pattern formed between radiation reflected from different points of the region; wherein the different points of the area may include some neighboring points and additional points.

The sensor may be constructed and arranged to sense a difference between an interference pattern associated with certain adjacent points and an interference pattern associated with additional points.

The small portion may not exceed ten percent.

The aperture may be located at the center of the aperture stop.

The aperture may be located outside the center of the aperture stop.

The aperture stop may be a removable part of the illumination optics in the inspection system.

The imager may be constructed and arranged to acquire a plurality of defocused images of the region under different defocused conditions.

The processor may be constructed and arranged to select a selected defocus condition of the different defocus conditions and use the selected defocus condition to acquire a defocused image of the region of the other object.

The processor may be constructed and arranged to select based on the contrast associated with the phase shift.

Detailed description of the invention

Because the apparatus implementing the present invention is, in most cases, composed of optical components and circuits known to those skilled in the art, circuit details will not be explained in any greater extent than that considered necessary as illustrated above, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention.

In the following description, the present invention will be described with reference to specific examples of embodiments of the present invention. It will, however, be evident that various modifications and changes may be made thereunto without departing from the broader spirit and scope of the invention as set forth in the appended claims.

A method and system are provided for achieving optical contrast enhancement of such defects by introducing modifications in the optical path.

The method and system enable defect inspection of semiconductors by using contrast-enhanced 2D optical imaging of defects.

A microscope with bright field imaging options for semiconductor defect inspection was introduced, with special means and methods to achieve visualization of "transparent defects".

Such defects cannot be detected by standard bright-field or dark-field imaging techniques. However, they do introduce a small phase shift of the light with respect to the background.

Defects of this type include bubbles, fine scratches, slip lines in the epitaxial layer, stress defects, and the like.

The method and/or system may implement at least one of the following:

a. Creating collimated light with high spatial coherence impinging on the object plane. The light is collimated when it impinges on the object. The collimated light may be fully collimated (fully parallel beamlets) but may provide a small deviation from fully parallel, e.g. a deviation from fully parallel of a few degrees, e.g. a deviation of 1-2 radians (since the full aperture of the objective lens may not exceed 10 radians). The allowable deviation may be a fraction (e.g., approximately up to 1,2, 3,4, 5,6, 7, 8, 9, and 10 percent) of the NA of the objective lens. The relationship between the area of the aperture size and the aperture stop size provides an indication of collimation.

B. The best defocus achieved by the special optical setup of the standard bright-field microscope is found for the creation of constructive or destructive interference of defects and background in the semiconductor.

C. the defocus level is controlled to obtain an optimized interference contrast.

D. defocus levels with positive or negative offsets are determined to control destructive or constructive interference.

E. The introduction of collimated light with an aperture stop of small diameter is achieved at the Kohler (Kohler) illumination input.

Different aperture stops of different shapes may be selected, for example decentering the opening at different distances, to enable the angle of incidence to be changed.

The microscope mode can be easily interchanged between standard bright-field imaging and contrast enhancement modes.

The method can be used to distinguish a flat smooth surface of a semiconductor wafer from a roughened surface of the same wafer so that surface haze can be analyzed.

To cope with low contrast features in an image by improving the contrast of the features, they are otherwise not visible by standard bright-field or dark-field imaging. The method creates collimated light with high spatial coherence that impinges on the object plane. And find the best defocus for creating constructive or destructive interference of the defect and background.

To cope with the complexity and expense of introducing phase contrast enhancement components in the optical path, it is made simpler by allowing modifications to be introduced in the standard optical path. The method may create collimated illumination, which may be accomplished by introducing a special aperture stop in the kohler illumination to create collimated light.

One such solution is to use a small circular diaphragm.

Fig. 1 shows a system 10 comprising an illumination path 20 providing light 11, which light 11 passes through an aperture of a beam stop 21 (also referred to as a shape controlled aperture stop, which is located at an aperture stop plane 91) to form a light beam 12, proceeds towards a beam splitter 22, is directed (by the beam splitter 22) to an objective lens 23 (the objective lens has an exit pupil plane 92) and passes through the objective lens 23 and a wafer (100) located in an object plane 93.

Defocus changes the focus of light from the object plane.

Light from the wafer (reflected light) passes through 23 the objective lens (and through the objective lens exit pupil plane), through the beam splitter 22, toward the tube lens 24 and onto the camera 30 located in the image plane 95. The camera 30 is coupled to a processor 40.

The illumination path 20 comprises a beam stop 21, a beam splitter 22 and an objective lens 23.

The collection path 29 includes the beam splitter 22, the objective lens 23, and the tube lens 24.

It should be noted that while fig. 1 shows an inspection system with a beam splitter, a bright field may be achieved without a beam splitter, for example, by illuminating an object from one side and collecting light from the other side.

It should be noted that the inspection system may also change the polarization of the illumination beam and/or the reflected beam by introducing a polarizing element (e.g. a polarizer) in the illumination path, before the illumination path or after the illumination path. See, for example, polarizer 28 in fig. 2 after the beam stop.

Fig. 3 shows various examples of the aperture stop 21.

The aperture stop 21 shown at the top of the figure includes an aperture 221 located at the center of the aperture stop 21.

The aperture stop 21 shown at the middle of the figure includes an aperture 221 located at the outside of the center of the aperture stop 21.

The aperture stop 21 shown at the bottom of the figure comprises an aperture 221 and an additional aperture 222.

The position of the aperture stop may determine the illumination angle of the area of the object. The apertures in different positions can intensify defects that are oriented at different orientations (typically at orientations perpendicular to the illumination angle).

Illuminating the same area with apertures at different locations within the aperture stop may provide information about the phase shift caused by defects oriented at different angles.

The collimated light may be obtained by using an aperture stop having one or more openings in the aperture stop, each opening having an area much smaller than the opening of the objective exit pupil plane (e.g., the radius of the opening is about ten to fifteen percent of the radius of the opening of the objective exit pupil plane).

When there are multiple apertures, the radius of each aperture may be a fraction of the radius of the aperture stop. Alternatively, the total radius of all apertures may be a fraction of the radius of the aperture stop.

The apertures may not be circular and in this case their size (area) is a fraction of the size (area) of the aperture stop.

The aperture stop 21 may be selectively inserted into the illumination path. Insertion may be done manually or automatically. The aperture stop 21 may be replaced by a non-blocking aperture that keeps the aperture plane clear of any obstructions (and thus enables the inspection system to operate in another mode (e.g., bright field mode). Note that differently configured aperture stops (different positions of the apertures, different sizes of the apertures, different numbers of apertures) may be replaced automatically or manually. The replacement may involve applying any movement-linear, nonlinear, circular (see turret 28, which rotates to selectively insert aperture stop 21 or full aperture 27), which will selectively insert or remove aperture stop 21 into or from the illumination path.

Fig. 5 shows that there is a difference between (a) a first interference pattern IR12 77 generated by points (P1 and P2) adjacent to each other but exhibiting the same optical path and (b) a second interference pattern IR34 78 generated by points (P3 and P4) adjacent to each other but exhibiting another optical path.

In the focal plane, the radiation from adjacent points does not overlap.

Due to the defocusing, the first light beam B1 from the first point P1 overlaps the second light beam B2 from the second point P2, thereby generating the first interference pattern IR12 77.

Due to the defocusing, the third light beam B3 73 from the third point P3 overlaps the fourth light beam B4 74 from the fourth point P4, thereby generating the second interference pattern IR34 78.

First interference pattern IR12 77 is different from second interference pattern IR34 78. In particular, the cumulative intensity of the first interference pattern IR12 77 sensed by the camera (in particular by the pixels of the camera) is different from the cumulative intensity of the second interference pattern IR34 78 sensed by the camera (in particular by the pixels of the camera). Thus, the gray level of the background pixel is expected to be different from the gray level of the pixel at the boundary of the defect.

Fig. 6 illustrates a method 200 for inspecting an object.

The method 200 may begin with the step 210 of acquiring a defocused image of a region of an object.

Step 220 may be followed by step 220 of processing the defocused image of the region to find a phase shift between optical paths associated with certain neighboring points of the region; wherein the phase shift indicates a defect.

The phase shift causes the interference pattern to be different from the interference pattern that would be caused without the phase difference. Phase differences may be introduced due to differences in height or other topography and/or differences in refractive index of points that are close to each other.

Step 210 may include illuminating the region with a radiation beam that is spatially coherent and collimated when impinging on the region. Illumination may include passing the radiation beam through an aperture defined by an aperture stop located in an aperture stop plane. The size of the aperture is a fraction of the size of the aperture stop.

The fraction may not exceed 1% to 2% and 2.5%. This may be achieved, for example, when the aperture radius is between about 10% and 15% of the radius of the aperture stop.

The aperture may be located at or outside the center of the aperture stop.

Step 210 may include sensing, by a sensor, sensed radiation reflected by the area; wherein the sensed radiation comprises an interference pattern formed between radiation reflected from different points of the area. The different points of the area may include some neighboring points and additional points.

Step 220 may include sensing a difference between an interference pattern associated with certain neighboring points and an interference pattern associated with additional points.

The aperture stop may be selectively inserted into and removed from the illumination path. The aperture stop may be brought into the illumination path prior to acquisition of the defocused image, and may be removed from the illumination path after acquisition of the defocused image.

Steps 210 and 220 may be performed multiple times while changing at least one configuration (changing defocus conditions such as distance between a portion of the inspection system and the object, changing aperture of the aperture stop, changing the aperture stop, and the like).

Multiple iterations of steps 210 and 220 may be performed until any stop condition is met, e.g., until a predetermined number of different configurations are checked, until a sufficiently good contrast is achieved, until a defect is found, and so on.

Multiple iterations may exhibit different defocus parameters, and each of one or more iterations may be performed using the new selected defocus conditions. A defocus condition is selected. The selection may be based on a contrast associated with the phase shift. Thus, for example, a defocus condition is found that provides the highest contrast (or at least above a contrast threshold) between the defect and its surroundings.

The method 200 may be performed using an aperture stop comprising a plurality of apertures. In this case, the illumination may include one or more additional apertures that pass the radiation beam through the aperture and through the aperture stop.

Furthermore, those skilled in the art will recognize that boundaries between the functionality of the above described operations are merely illustrative. The functionality of multiple operations may be combined into a single operation, and/or the functionality of a single operation may be distributed in additional operations. Further, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments.

Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. In an abstract, but still definite sense, any arrangement of components to achieve the same functionality is effectively "associated" such that the desired functionality is achieved. Thus, any two components herein combined to achieve a particular functionality can be seen as "associated with" each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being "operably connected," or "operably coupled," to each other to achieve the desired functionality.

However, other modifications, variations, and alternatives are also possible. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.

The word "comprising" does not exclude the presence of other elements or steps than those listed in a claim. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein.

Furthermore, the terms "a" or "an," as used herein, are defined as one or more than one. Furthermore, the use of introductory phrases such as "at least one" and "one or more" in a claim should not be construed to imply that the introduction of another claim element by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim element to inventions containing only one such element, even if the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an". The same applies to the use of definite articles. Unless otherwise indicated, terms such as "first" and "second" are used to arbitrarily distinguish between the elements such terms describe.

Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method for inspecting an object, the method comprising:

acquiring a defocused image of a region of the object; and

processing a defocused image of the region to find a phase shift between light paths associated with certain adjacent points of the region; wherein the phase shift is indicative of a defect;

wherein acquiring the defocused image comprises illuminating the region with a radiation beam, the radiation beam being spatially coherent and collimated when impinging on the region;

wherein the illuminating comprises passing the radiation beam through an aperture defined by an aperture stop, the aperture stop being located in an aperture stop plane; and

Wherein, the size of the aperture is a small fraction of the size of the aperture stop, and the small fraction does not exceed 2.5%.

2. A method according to claim 1, wherein the acquisition of the defocused image of the area includes sensing, by a sensor, sensed radiation reflected by the area; wherein the sensed radiation includes an interference pattern formed between radiation reflected from different points of the area; wherein the different points of the area include certain adjacent points and additional points.

3. The method of claim 2, comprising sensing a difference between an interference pattern associated with the certain adjacent points and an interference pattern associated with the additional points.

The method of claim 1 , wherein the aperture is located at a center of the aperture stop.

5. The method of claim 1, wherein the aperture is located outside of a center of the aperture stop.

6 . The method of claim 1 , wherein the aperture stop is brought into an illumination path prior to acquisition of the defocused image and the aperture stop is removed from the illumination path after acquisition of the defocused image.

7. The method of claim 1, comprising acquiring a plurality of defocused images of the area under different defocus conditions.

8. The method of claim 7, comprising selecting a selected one of the different defocus conditions and using the selected defocus condition to acquire a defocused image of a region of a further object.

9. The method of claim 8, wherein the selecting is based on a contrast ratio associated with the phase shift.

10. The method of claim 1, wherein the illuminating further comprises passing the radiation beam through an additional aperture defined by the aperture stop, wherein the aperture is located at a center of the aperture stop and the additional aperture is located outside the center of the aperture stop.

11. An inspection system comprising:

an imager constructed and arranged to acquire a defocused image of a region of the object; and

a processor constructed and arranged to process the defocused image of the region to find phase shifts between optical paths associated with certain adjacent points of the region; wherein the phase shifts are indicative of a defect;

wherein the imager comprises illumination optics constructed and arranged to illuminate the area with a radiation beam, the radiation beam being spatially coherent and collimated when impinging on the area;

wherein the illumination optics comprises an aperture stop, the aperture stop comprising an aperture, wherein the illumination optics is constructed and arranged such that the radiation beam passes through the aperture before reaching the region; and

12. An inspection system according to claim 11, wherein the imager includes a sensor constructed and arranged to sense sensed radiation reflected by the area; wherein the sensed radiation includes an interference pattern formed between radiation reflected from different points of the area; wherein the different points of the area include the certain adjacent points and additional points.

13. The inspection system of claim 12, wherein the sensor is constructed and arranged to sense a difference between the interference pattern associated with the certain adjacent points and the interference pattern associated with the additional points.

14. The inspection system of claim 11, wherein the aperture is located at a center of the aperture stop.

15. The inspection system of claim 11, wherein the aperture is located outside of a center of the aperture stop.

16. The inspection system of claim 11, wherein an aperture stop is a removable portion of the illumination optics.

17. The inspection system of claim 11, wherein the imager is constructed and arranged to acquire a plurality of defocused images of the area under different defocus conditions.

18. The inspection system of claim 17, wherein the processor is constructed and arranged to select a selected defocus condition from among the different defocus conditions and to acquire a defocused image of a region of another object using the selected defocus condition.

19. The inspection system of claim 17, wherein the processor is constructed and arranged to make a selection based on a contrast associated with the phase shift.

20. The inspection system of claim 11, wherein the aperture stop includes an additional aperture, wherein the aperture is located at a center of the aperture stop and the additional aperture is located outside of a center of the aperture stop.